I was a bit skeptical at first but look at it this way, the median price for units in my building is about $750K and the average price for a car space is about $60K. I would put the value of non-car storage space at about one-third to a quarter of that which would work out between two and three percent.
Another way to look at it is that eight cubic metres of commercial storage space in the Eastern Suburbs will cost about $230 a month. It may differ in your area and you wouldn’t be charging commercial rates but it does give you an idea of the intrinsic value. Have a look at the Kennards website to make your comparison for your area and see how it adds up.
One thing I do know, you don’t realise how valuable storage space is until you move into an apartment that doesn’t have any. I think the pro-storage people may be gilding the lily a little, but they do have a point.